High efficiency group III nitride LED with lenticular surface

ABSTRACT

A light emitting diode is disclosed that includes a conductive substrate, a bonding metal on the conductive substrate and a barrier metal layer on the bonding metal. A mirror layer is encapsulated by the barrier metal layer and is isolated from the bonding metal by the barrier layer. A p-type gallium nitride epitaxial layer is on the encapsulated mirror, an indium gallium nitride active layer is on the p-type layer, and an n-type gallium nitride layer is on the indium gallium nitride layer, and a bond pad is made to the n-type gallium nitride layer.

This application is a continuation of Ser. No. 11/082,470 filed Mar. 17,2005 for “High Efficiency Group III Nitride LED with LenticularSurface,” and now published as No. 20060060874. Ser. No. 11/082,470 is acontinuation-in-part of Ser. No. 10/951,042 filed Sep. 22, 2004 for“High Efficiency Group III Nitride Silicon Carbide Light EmittingDiode,” and now U.S. Pat. No. 7,259,402. The contents of No. 20060060874and of U.S. Pat. No. 7,259,402 are incorporated entirely herein byreference.

BACKGROUND

The present invention relates to light emitting diodes (“LEDs”) and inparticular relates to high-brightness light emitting diodes formed fromGroup III nitride active structures on silicon carbide substrates.

This application is also related to the following copending and commonlyassigned U.S. applications: Ser. No. 10/811,350 filed on Mar. 26, 2004for, “Etching of Substrates of Light Emitting Devices,” and now U.S.Pat. No. 7,202,181; Ser. No. 60/591,353 filed on Jul. 27, 2004 for,“Ultra-Thin Ohmic Contacts for P-Type Nitride Light Emitting Devices,”and now published as No. 20060046328; and Ser. No. 60/639,705 filed Dec.28, 2004 for, “Ultra-Thin Ohmic Contacts for P-Type Nitride LightEmitting Devices,” now published as No. 20060046328. The contents ofeach of these are incorporated entirely herein by reference.

The semiconductor era has witnessed the replacement of many types ofelectrical devices with solid state counterparts. The most obvious isperhaps the replacement of the vacuum tube (almost unknown to presentyounger generations) with the transistor. Solid state devices, becauseof their nature and operation, are inherently much more reliable thanearlier generations of electronic devices and can have significantlylonger lifetimes, typically by a factor of at least 100. In comparisonto such mature technologies, solid-state devices are longer-lasting,physically more rugged, use less power, and are more efficient.

A light emitting diode (LED) is a p-n junction semiconductor diode thatemits light when current is injected across a p-n junction (forwardbiased) to drive the recombination of electrons and holes with theconcurrent production of photons. Thus, light emitting diodes producelight based upon the movement of electrons in a semiconductor material.Therefore, LEDs do not require (although they can be used in conjunctionwith) vapors or phosphors. They share the desirable characteristics ofmost semiconductor-based devices, including high efficiency (theiremissions comparatively little heat), high reliability and long life.For example, typical LEDs have a mean time between failures of betweenabout 100,000 and 1,000,000 hours meaning that a conservative halflifetime for an LED is on the order of 50,000 hours.

An LED's emitted light has a frequency (which in turn relates directlyto wavelength and color in accordance with well-understood principles ofphysics) based upon the energy difference between permitted energylevels in the material, a characteristic referred to as the bandgap. Thebandgap is a fundamental property of the semiconductor material and itsdoping. For example, gallium arsenide phosphide (GaAsP) represents awell-established material system for light emitting diodes. Depending onthe mole fraction of Ga and As, these materials have a bandgap ofbetween about 1.42 and 1.98 electron volts (eV), and will emit light inthe infrared, red and orange portions of the electromagnetic spectrum.

In order to further commercialize light emitting diode applications,however, colors other than red, orange and yellow must be available.Specifically blue and green light emitting diodes are required (alongwith red diodes) to create white light or full color displays. Becausethese colors represent higher-energy portions of the visible spectrum,they require larger transitions than the bandgaps of silicon or galliumarsenide can provide.

In turn, because green, blue, and ultraviolet (UV) photons representhigher frequency colors (E=hυ) within (and beyond) the visible spectrum,they can only be produced by LEDs with bandgaps of at least about 2.2eV. Such materials include diamond (5.47 eV), silicon carbide (2.99 eV)and Group III nitrides such as GaN (3.4 eV). In addition to producinggreen, blue or ultraviolet light per se, wide bandgap LEDs can becombined with red and green LEDs to produce white light, or withphosphors that produce white light when excited by blue or UV light, orboth.

For several reasons, the Group III nitride compositions (i.e., Group IIIof the periodic table), particularly GaN, A;GaN, InGaN and AlInGaN areparticularly useful for LEDs that emit in the ultraviolet (UV) throughgreen portions of the spectrum. As one advantage, they are “direct”bandgap materials, meaning that when an electron transition occursacross the bandgap, much of the energy is emitted as light. Bycomparison, “indirect” materials (such as silicon carbide) emit theirenergy partially as light (a photon) and predominantly as vibrationalenergy (a phonon). Thus Group III nitrides offer efficiency advantagesover indirect transition materials.

As another advantage, the bandgap of ternary and quaternary Group IIImaterials (e.g., AlGaN, InGaN, and AlInGaN) depends upon the atomicfraction of the included Group III elements. Thus the wavelength (color)of the emission can be tailored (within limits) by controlling theatomic fraction of each Group III element in a ternary or quaternarynitride.

Wide bandgap semiconductors have been, however, historically moredifficult to produce and work with than gallium-arsenide or galliumphosphide (GaP). As a result, blue and UV-emitting LEDs have laggedbehind GaP-based LED's in their commercial appearance. For example,silicon carbide is physically very hard, has no melt phase, and requireshigh temperatures (on the order of about 1500-2000° C.) for epitaxial orsublimation growth. The Group III nitrides have relatively largenitrogen vapor pressures at their melting temperatures and thus arelikewise difficult or impossible to grow from a melt. Additionally,difficulties in obtaining p-type gallium nitride (and other Group IIInitrides) remained a barrier to diode production for a number of years.Accordingly, the commercial availability of blue and white-emitting LEDsis more recent than the corresponding availability of GaP-based andGaAs-based LEDs.

A number of commonly assigned patents and co-pending patent applicationslikewise discuss the theory and nature of light emitting diodes,including but not limited to U.S. Pat. Nos. 6,459,100; 6,373,077;6,201,262; 6,187,606; 5,912,477; 5,416,342; and 5,838,706; and PublishedU.S. Applications Nos. 20020022290; 20020093020; and 20020123164. Thecontents of these are incorporated entirely herein by reference.

Blue LEDs and their related derivative devices are becoming morefrequently included in consumer electronic devices, particularly smalldisplays. Common examples include items such as computer screens,personal digital assistants (“PDAs”) and cellular phones. In turn, thesesmall devices drive demand for thinner LEDs with reduced area(“footprint”). Such LEDs, however, must still operate at low forwardvoltages (Vf) and high light output. To date, however, reducing the sizeof the Group III nitride devices has tended to increase their forwardvoltage and reduce their radiant flux.

In addition to providing blue, green, or white light (as well asemissions in the ultraviolet range), the Group III nitride lightemitting diodes have the potential to provide replacement forlong-standing illumination technologies such as incandescent andfluorescent lighting. Historically, however, LEDs have lacked brightnesscomparable to incandescent, fluorescent or vapor-discharge lights andthus these older technologies have continued to occupy the field. Onlyrecently, have white LEDs (or LED-based white-emitting devices) begun tomake inroads into commercial lighting applications, with most of thesebeing in smaller applications such as flashlights and related items.

In commercial embodiments of light emitting diodes (e.g., the XBRIGHT™diodes offered by the assignee herein; Cree, Inc.; Durham, N.C.) recentadvances have included an inverted device design. U.S. Pat. No.6,740,906 discusses aspects of this design as does U.S. PatentApplication Publication No. 20020123164. The contents of both of theseare incorporated entirely herein by reference. In this type of design,the Group III active layers are grown (typically epitaxially) on asilicon carbide substrate. Light emitting diodes of this type are thenmounted with their epitaxial layers (“epilayers”) “down” rather than“up”; i.e., the silicon carbide portions form the display face of themounted device. In this orientation the epitaxial layers are mounted toand face a circuit or “lead frame” that provides the electricalconnection to the diode. The silicon carbide-up orientation increaseslight extraction from the device as set forth in the '906 patent and the'164 publication.

Silicon carbide can also be conductively doped. This provides advantagesin comparison to sapphire based Group III nitride diodes. Becausesapphire is an insulator, two top wire bonds are typically required tomount a working sapphire-based diode. In comparison, silicon carbidedevices can be “vertically” oriented; i.e., with ohmic contacts onopposite ends of the device. Such vertical orientation is directlyanalogous to diodes formed in other conductive semiconductor materialssuch as gallium arsenide (GaAs), and thus the same mounting orientationsand techniques can be used.

Although these “inverted” devices have successfully provided significantpractical and commercial improvements, their “epilayer-down” orientationrequires different, and to some extent more sophisticated, mounting onlead frames. In particular, because the active portion (p-n junction,multiple quantum well, etc.) is positioned closely adjacent to the leadframe, avoiding short circuits or other undesired interactions betweenthe active portion and lead frame becomes more difficult.

For example, conventional LEDs (including Group III nitride on SiCdevices) are often mounted on the lead frame using conductive silverepoxy. Silver epoxy is a mixture of more than about 50 percent by weightof microscopic silver particles with epoxy resin precursors. When usedto connect electronic devices to circuits (or circuit boards) the silverepoxy provides flexibility, relative ease of handling, conductivity andgood heat transfer properties. Because silver epoxy is (purposely)applied as a viscous liquid, it can and will flow accordingly and,unless other steps are taken, will tend to surround lower portions ofany diode mounted with it. As noted above, if the active portions areadjacent the lead frame, the flowing silver epoxy can contact the activeportion and cause short circuiting or other undesired interactions.

As a result, many conventional light emitting diode mounting techniquesare too difficult, too unreliable or simply unavailable for invertedGroup III nitride silicon carbide devices. Other specific techniques(e.g., copending application Ser. No. 10/840,463 filed May 5, 2004should or must be incorporated to avoid these problems.

As another potential solution, the inverted device can be positioned onsome sort of sub-structure, with the sub-structure being attached to thelead frame. Examples include U.S. Patent Application Publication No.20030213965. The sub-structure is included to add sufficient thicknessto remove the active portions farther from the lead frame and its silverepoxy or related materials. As set forth in No. 20030213965, however,soldering the device to a substructure can undesirably tilt the devicewith respect to the sub-structure and thereby exacerbate theshort-circuiting problem.

Accordingly, it remains a continuing goal to increase the currentcapacity, light output (power) and light extraction (geometry)capabilities of inverted light emitting diodes while concurrentlyreducing their size and particularly reducing their thickness. Itremains a similar goal to produce such diodes in designs that can beconveniently incorporated into lead frames, packages and circuits in amanner similar or identical to related diodes.

SUMMARY

In one aspect the invention is a high efficiency Group III nitride lightemitting diode that includes a substrate selected from the groupconsisting of semiconducting and conducting materials, a Group IIInitride-based light emitting region on the substrate; and a lenticularsurface containing single crystal silicon carbide on the light emittingregion.

In another aspect the invention is a high efficiency Group III nitridelight emitting diode that includes a conductive silicon carbidesubstrate; an aluminum indium gallium nitride light emitting region onthe substrate, and a thin lenticular surface of silicon carbide on thelight emitting region.

In another aspect, the invention is a high efficiency Group III nitridelight emitting diode that includes a substrate selected from the groupconsisting of conducting and semiconducting materials, at least onemetal bonding layer on one surface of the substrate, an ohmic contact tothe opposite surface of the substrate from the metal layer, a lightemitting structure based upon the Group III nitride system on the atleast one metal layer on the substrate, an ohmic contact on the lightemitting structure opposite the substrate, and a lenticular surface, atleast portions of which are formed of silicon carbide, on the portionsof the light emitting structure other than the ohmic contact.

In another aspect, the invention is a high efficiency Group III nitridelight emitting diode that includes a silicon carbide substrate, abackside ohmic contact on one surface of the substrate for providing oneelectrical connection to the diode, a metal bonding layer on theopposite surface of the substrate for providing a physical andelectronic transition between the substrate and other portions of thediode, a mirror layer on the metal bonding layer for enhancing lightextraction from the diode, a p-type Group III nitride layer on themirror, a light emitting Group III nitride layer on the p-type layer andhaving a bandgap smaller than the bandgap of the p-type layer, an n-typeGroup III nitride layer on the light emitting layer and having a bandgaplarger than the bandgap of the light emitting layer, a wire bond padohmic contact to the n-type layer for providing a second electricalconnection to the diode, and a lenticular surface, at least portions ofwhich are formed of silicon carbide, on the portions of the n-type layerother than the wire bond pad.

In another aspect, the invention is a high-efficiency Group III nitridelight emitting diode package that includes a lead frame, and a lightemitting diode on the lead frame, the diode including, a substrateselected from the group consisting of semiconducting and conductingmaterials, a Group III nitride based light emitting region on thesubstrate, a lenticular surface containing single crystal siliconcarbide on the light emitting region, an ohmic contact between thesubstrate and the lead frame, and an ohmic contact to the light emittingregion.

In another aspect, the invention is a wafer structure for highefficiency Group III nitride light emitting diode precursors thatincludes a conductive silicon carbide substrate wafer, a Group IIInitride epitaxial layer on the substrate, a plurality of discrete ohmiccontacts on the surface of the Group III nitride epitaxial layer, theohmic contacts defining a plurality of discrete light emitting diodeprecursors, and a lenticular surface containing single crystals siliconcarbide on the Group III nitride epitaxial layer.

The foregoing and other objects and advantages of the invention and themanner in which the same are accomplished will become clearer based onthe followed detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional schematic illustration of a diode accordingto the present invention.

FIG. 2 is a cross sectional schematic illustration of a secondembodiment of a diode according to the present invention.

FIG. 3 is a cross-sectional schematic illustration of a third embodimentof a diode according to the present invention.

FIG. 4 is a schematic cross-sectional illustration of a diode and a leadframe in accordance with the present invention.

FIG. 5 is a cross sectional schematic diagram of a semiconductor waferin accordance with the present invention.

FIG. 6 is a scanning electron microscope (SEM) photograph of a diodeaccording to the present invention.

FIGS. 7 and 8 are SEM photographs of the surface of a diode according tothe present invention.

FIG. 9 is an SEM photograph taken cross-sectionally across a diodeaccording to the present invention.

FIG. 10 is a schematic cross-sectional view of another embodiment of adiode according to the present invention.

FIG. 11 is a sectional view taken along lines 11-11 of FIG. 10.

FIGS. 12 and 13 are additional cross-sectional views of additionalembodiments of diodes according to the present invention.

FIG. 14 is a cross-sectional view of a diode with a mesa structureaccording to the present invention.

DETAILED DESCRIPTION

In a first aspect, the invention is a high efficiency Group III nitridelight emitting diode. FIG. 1 is a schematic illustration taken in crosssection of a diode according to the present invention with the diodebroadly designated at 20. The diode includes a substrate 21 selectedfrom the group consisting of semiconducting and conducting substrates, aGroup III nitride based light emitting layer 22 on or above thesubstrate 21, and a lenticular surface 23 that contains silicon carbideon or above the light emitting layer 22. These Group III nitride layerstogether define a light emitting region 37.

In most embodiments, the silicon carbide of the lenticular surface 23 issingle crystal. It will be understood by those familiar with siliconcarbide and crystal structure that the descriptive term “single crystal”refers to the silicon carbide material and is entirely consistent withthe lenticular surfaces described herein that obviously include a numberof individual structures, each of which is formed of single crystalsilicon carbide. The substrate can likewise be single crystal siliconcarbide, but can also be selected from other materials such as silicon(Si), gallium arsenide (GaAs) or copper-tungsten (Cu—W)

As used in the context of the invention, the description of layers ofmaterials “on” one another can refer to either layers that are above oneanother (with something in between) and layers that are directly incontact with one another. Each meaning will be clear in context,particularly to those persons of ordinary skill in this art.

As further illustrated in FIG. 1, the diode includes respective ohmiccontacts 24 to the substrate 21 and 25 to the light emitting region 37or to the epitaxial layer 27. As is typical in many diodes of this type,the backside ohmic contact 24 is relatively large for the purpose ofencouraging current flow through the diode 20 while the ohmic contact 25to the light emitting region 37 is typically smaller and in the form ofa wire bond pad for minimizing the degree to which the ohmic contact 25blocks light from leaving the diode.

FIG. 1 also illustrates that in several embodiments, the diode willinclude both p-type and n-type layers of Group III nitride for currentinjection across the p-n injunction and encouraging the recombination ofcarriers in the light emitting layer 22. FIG. 1 illustrates such layersas an n-type layer 27 and a p-type layer 26. Accordingly, FIG. 1 showsthree Group III nitride layers (22, 26, and 27) but in some embodiments(not illustrated) only one p-type layer and one n-type layer areincluded.

Thus, in embodiments for which FIG. 1 is illustrative, the preferredGroup III nitride material is aluminum indium gallium nitride(Al_(x)In_(y)Ga_(1-x-y)N where 0≦x, y≦1 and x+y≦1) and the atomicfractions of aluminum, indium and gallium in each layer are selected sothat light emitting layer 22 has a bandgap smaller than either of theadjacent layers 26 or 27.

The adjustment of the atomic fractions (i.e., x and y) of the Group IIIelements to obtain the desired bandgap is well understood in this artand can be carried out without undue experimentation. In addition toselecting the atomic fractions on the basis of the desired bandgap,other factors are typically considered such as crystal structure,stability, and the desired or necessary manufacturing steps.

The lenticular features described and claimed herein can be formed usingat least the techniques set forth in copending and commonly-assignedapplication Ser. No. 10/815,293 filed Apr. 1, 2004 for, “LaserPatterning of Light Emitting Devices and Patterned Light EmittingDevices,” and now U.S. Pat. No. 7,419,912, the contents of which areincorporated entirely herein by reference. Other techniques will beapparent to those of ordinary skill in this art.

In preferred embodiments, the substrate 21 is formed of single crystalsilicon carbide (SiC) and is formed of one of the 3C, 4H, 6H and 15Rpolytypes of silicon carbide. Although the invention is not limited tothese polytypes, they have favorable characteristics of thermalexpansion and thermal conductivity.

The substrate 21 can also be a sub-mounting structure as set forth incommonly assigned U.S. Pat. No. 7,259,402. In either case (siliconcarbide substrate or sub-mounting structure substrate), a metal bondinglayer 30 can be favorably incorporated, typically for the purpose ofmanufacturing steps as set forth in the above U.S. Pat. No. 7,259,402.The metal layer 30 preferably includes a metal that forms ahigher-temperature bond (e.g. above about 260° C.) which is helpful (orin some cases necessary) for lead-free assembly processes. In presentlypreferred embodiments the metal layer 30 uses gold (Au) and tin (Sn)either as a eutectic blend or as individual layers.

As further set forth in the U.S. Pat. No. 7,259,402, the diode 20 canalso include a mirror layer 31 for enhancing the light extraction fromthe emitting layer 22. The mirror and the metal bonding layer can beselected of any appropriate metals provided they are conductive andotherwise consistent (do not interfere) with the structure and functionof the remainder of the diode 20 and its elements. As set forth in theincorporated references, if the mirror is made of silver (Ag),additional structural layers of metal may be optionally included toprevent migration of silver into adjacent layers while still takingadvantage of its reflective properties. Such barrier layers aretypically (but not exclusively) formed of a titanium-tungsten (TiW)alloy, a nitride of the TiW alloy, or as a composite of TiW and platinum(Pt).

FIG. 2 illustrates another embodiment of the diode broadly designated at28. Many of the elements of FIG. 2 are identical to those in FIG. 1 andthus they retain the same reference numerals and will not bere-described herein. As FIG. 2 particularly indicates, however, thelenticular surface, now designated at 32, extends to, but not beyond,the light emitting region 37.

Thus, in FIG. 2 the lenticular surface 32 is illustrated as upon then-type Group III nitride layer 27 rather than directly on the lightemitting layer 22. Stated differently, in FIG. 1 the lenticular siliconcarbide surface 23 includes at least some non-lenticular silicon carbidebetween the lenticular features and the remainder of the diode 20. InFIG. 2, the lenticular features are immediately upon the remainder ofthe diode with no silicon carbide in between. In either embodiment (FIG.1 or FIG. 2) the lenticular features 23, 32 can be directly adjacent oneanother or they can be spaced from one another with non-lenticularportions therebetween.

FIG. 3 illustrates yet another embodiment of the diode broadlydesignated at 34. Once again, elements common to FIGS. 1 and 2 carry thesame reference numerals. As a further explanation and illustration, inall of the embodiments, the layers that together make up the lightemitting region, regardless of the number of layers, can be broadlyconsidered as a unit which is labeled at 37 in FIG. 3. Thus, in FIGS.1-4 herein, a light emitting layer is designated at 22 while a lightemitting region (which includes the emitting layer 22) is designated at37. Those familiar with light emitting structures will recognize thatthe light emitting region can be as simple as a p-n junction, or can beformed of a few layers (as illustrated), or of more complex structuressuch as superlattices and multiple quantum wells.

In FIG. 3, however, the lenticular features are defined both by siliconcarbide portions 35 and by features designated at 36 that extend intothe remainder of the diode 34. In particular, in the embodimentillustrated in FIG. 3 the lenticular features 35 and 36 extend into thelight emitting region 37, and because in this embodiment the diode 34includes the n-type epitaxial layer 27, the features 36 extend into suchlayer 27.

FIG. 4 is another cross-sectional schematic diagram of the diode 20 fromFIG. 1 positioned on a lead frame 40. Because the diode 20 in FIG. 4 isidentical to the diode 20 of FIG. 1, all of the elements carry the samereference numerals. It will be further understood that FIG. 4 is notdrawn to scale, but rather for schematic illustration purposes. Asillustrated in FIG. 4, the ohmic contact 24 is positioned between thesubstrate 21 and the lead frame 40 for providing an appropriateelectrical contact between the lead frame 40 and the diode 20. As inmany manufacturing techniques for mounting and packaging light emittingdiodes, a conductive paste 41 helps attach the diode 20 to the leadframe. As illustrated in FIG. 4 and as set forth in U.S. Pat. No.7,259,402, the substrate is of sufficient thickness to prevent theconductive paste 40 from touching (and thus electrically shorting) theGroup III nitride region 37 or the light emitting layer 22.

Furthermore, although FIG. 4 illustrates the diode 20 in relativelylarge proportional context, it preferably incorporates the structuraladvantages set forth in co-pending application Ser. No. 10/951,042 tocreate a total thickness between and including the ohmic contacts 24 and25 of no more than 150 microns. It will also be understood in thiscontext that the wire bond pad ohmic contact 25 is shown as beingpositioned lower than the top of the lenticular surface 23. In suchcases, the diode 20 would have a thickness no more than 150 micronsinclusive of the backside ohmic contact 24 and the lenticular features23.

It will be understood that FIGS. 1 through 4 are schematic rather thanlimiting of the diodes according to the claimed invention. For example,those familiar with diodes of this type will recognize that portions ofthe diodes, often including the light emitting region, are formed asmesas rather than with entirely vertical sides illustrated in FIGS. 1-4.Additionally, the illustrated structures or related mesa structures cantypically include passivation layers for various well-understoodpurposes. Because the nature of the invention is such that it can beincorporated with or without mesa structures and with or withoutsupplementary passivation, these alternative structures have not beenspecifically illustrated, but fall within the scope of the claims.

FIG. 5 shows the invention in the context of a wafer structure. In onesense, the wafer structure can be an extension of the individual diodesthat are formed from the wafer. Thus, FIG. 5 illustrates a wafer broadlydesignated at 43 that includes a substrate 44, preferably (but notnecessarily) a single crystal n-type silicon carbide substrate, a lightemitting region 37 that can include one or more light emitting layers orcombination of layers that together function to emit light when currentis injected through them, and a plurality of discrete ohmic contacts 45that define a plurality of individual diodes broadly designated at 46and separated by the guidelines 47. The wafer includes the lenticularsilicon carbide surface features designated at 50 on the Group IIInitride active region 37.

FIGS. 6 through 10 are scanning electron microscope (SEM) photographs ofdiodes and portions of diodes according to the present invention.

FIG. 6 is a perspective view of a single light emitting diode accordingto the present invention. Both the lenticular surface and the top wirebond pad are clearly visible on top of a substrate which is best shownin other of the photographs.

FIGS. 7 and 8 illustrate that the lenticular surface can comprise aplurality of cones. The features may be more accurately described ascone-like because in a geometry sense, a proper solid cone is generatedby rotating a right triangle about one of its legs. The slightly roundedshapes of the cones in FIGS. 7 and 8 are thus slightly different from asolid cone, but the description will be understood by those of skill inthe art and in context of the photographs. In other embodiments, thelenticular surface can be formed of a plurality of pyramids which arelikewise defined as a solid figure contained by planes which isconstructed from one plane to one point. Both four-sided and six sidedpyramids can be incorporated.

Other shapes for lenticular surfaces can be formed in an almost endlessvariety depending upon the masking and etching techniques used to formor generate the features. Thus, the invention is not limited to cones,cone-like features or pyramids, but will encompass other lenticularsurfaces that can be selected by those of skill in this art and withoutundue experimentation.

FIG. 9 is a cross-sectional view showing actual examples of some of thefeatures that were illustrated schematically in FIGS. 1-4.

FIG. 9 illustrates the lenticular portion at 52 (to differentiate fromthe schematic diagrams of FIGS. 1-4). The additional silicon carbideportion below the lenticular features 52 is designated at 53. The lightemitting region, including several layers, is designated at 54 and wouldcorrespond to region 37 in FIGS. 3 and 4. The substrate is illustratedat 55.

In comparison to other techniques and structures, the invention offersseveral advantages. For example, the various laser lift-off techniquesfor reducing the thickness of a substrate (and thus of the resultingdiode; e.g., Published Patent Application No. 20030197170) are difficultto carry out in a practical sense without inducing cracking of thewafer. Thus, the substrate removal grinding technique that is compatiblewith the present invention is expected to give higher yield.Additionally, laser lift-off releases nitrogen gas. In turn, nitrogenrelease is a major cracking mechanism in these materials systems.

As another benefit, the normally disadvantageous absorption in thesilicon carbide is effectively minimized because the volume of thelenticular shapes is likewise minimal.

As another advantage, the absorption coefficient of gallium nitride isgenerally higher than that of silicon carbide, with the proviso that theabsorbency (absorption) of a gallium nitride epitaxial layer cannot bedirectly compared to a bulk silicon carbide crystal.

As another advantage, gallium nitride will start to exhibit crackingproblems at thicknesses of between about 3 and 4 microns when siliconcarbide is used as the substrate and at about 5 microns when sapphire isused as a substrate. Thus, the use of the more robust SiC for thelenticular features offers structural advantages.

As another advantage, gallium nitride typically has a defect density inthe range of 10⁸ to 109 per cubic centimeter (cm-3). In turn, defectscan act as undesired absorption centers. In contrast, the typical defectdensity in silicon carbide is between about 103 and 104 cm⁻³.Accordingly, incorporating silicon carbide results in a better qualitycrystal that demonstrates lower defect absorption, which is advantageousfor the efficiency of a light emitting diode.

As another advantage, for a given thickness of n-type gallium nitride,the presence of silicon carbide always improves the current spreading.Additionally, silicon carbide gives the greatest latitude for machiningor otherwise forming the lenticular surface.

Silicon carbide also has a higher refractive index than gallium nitride,a factor that has no net disadvantage and may provide additionaladvantages.

FIG. 9 is a scanning electron microscope photograph takencross-sectionally of a portion of a diode according to the presentinvention. The various elements of the diode are labeled with thebracketed arrows adjacent the photograph. FIG. 9 accordingly illustratesthe lenticular surface 52, and the non-lenticular portion of siliconcarbide 53. The light emitting region is again designated at 37, themirror (and related barrier metals) at 31 and the metal bonding layer at30. A portion of the substrate is labeled 55 at the bottom of thephotograph. The five micron scale adjacent the photograph gives anindication of the relative sizes of the lenticular features and therelative thicknesses of the layers.

FIG. 10 is a cross-sectional schematic view of another embodiment of thepresent invention illustrating portions of a diode structure 60 thatenhances the amount of light extraction for a given amount of currentapplied across the diode. Where possible and helpful, the referencenumerals in FIG. 10 are the same as those used in FIGS. 1 through 4.Thus, FIG. 10 includes the wire bond pad 25 on the n-type Group IIInitride layer 27. FIG. 10 also illustrates the light emitting layer 22and the p-type Group III nitride layer 26. The lenticular features areagain indicated at 23.

The mirror layer, however, is now designated as 61. In the embodimentillustrated in FIG. 10, the mirror layer 61 is no longer a completelayer of the mirror metal (which also forms a portion of the electricalcontact to the active region 37). Instead, a portion of the mirror andcontact metals have been removed (or simply not included) to define anopening (designated at 62) adjacent the layer 26 beneath the geometricprojection of the wire bond pad 25 through the diode 60. Avoiding metalcontact at or over a geometric position projected from the wire bond pad25 helps direct current flow in the more transparent portions of thediode 60 between the mirror (and related metal) layer 61 and the wirebond pad 25. By eliminating a portion of the mirror 61, the designreduces the number of light-emitting recombinations occurring directlyunder the wire bond pad 25, and increases the number of recombinationsunder the more transparent portions of the diode 60. This in turnincreases the light extraction from the diode 60 for a given amount ofcurrent as compared to a diode in which the mirror and contact system 61completely covers one side (or the majority of one side) of the diode.

FIG. 11 is a top plan view of the diode 60 taken along lines 11-11 inFIG. 10. The layer 26 is again illustrated in the cross-hatched pattern,but the area not included in the mirror (e.g. an opening) is illustratedas the dark circle 62. The opening 62 is indicated with the darkershading because it has a darker appearance when the diode is constructedin the embodiment of FIG. 10.

FIG. 12 illustrates another embodiment of the diode designated at 64 inwhich the opening 62 is filled with a reflective metal 65 but one thatdoes not form an ohmic contact with the layer 26. As a result, lightextraction from the diode 64 is somewhat better than that of the diode60 illustrated in FIG. 10 because the mirror 61 includes more reflectivearea while still providing a structure that minimizes the number ofrecombinations and emitting events occurring directly under the wirebond pad 25.

FIG. 13 shows yet another embodiment of the diode designated at 66 inwhich a portion 70 of the p-type Group III nitride layer 26 ispassivated, preferably using a plasma, to be less conductive or eveninsulating. Because the passivation portion 70 discourages current flow,a full mirror layer 67 can be included in the diode 66 while stillserving to minimize the recombinations taking place directly under thewire bond 25.

It will be understood that although FIGS. 10-13 illustrate the opening62, the alternative metal portion 65, and the passivation portion 70 asbeing identical in size and geometry to the wire bond pad 25, variationsin the size of the opening 62, the alternative metal portion 65, or thepassivation layer 70 can be carried out for the same purpose withoutdeparting from the scope of the claimed invention.

FIG. 14 is another embodiment of a diode broadly designated at 70according to the present invention. The embodiment illustrated at 70 isvery similar to the embodiment illustrated in the other figures, withthe exception that the light emitting region 37 is in the form of a mesathat in turn includes a passivation layer 71 that covers otherwiseexposed portions of the mirror 31 (and any related encapsulating metals)and the bonding metal (or metals) layer 30. Because of the similarity tothe other figures, FIG. 14 includes identical reference numeralswherever appropriate. In typical embodiments, the passivation layercovers both the walls of the mesa and the silicon carbide lenticularsurface 23. Appropriate passivation materials included silicon nitride(both stoichiometric Si₃N₄ and non-stoichiometric) and silicon dioxide(SiO₂). It will be understood that these or other passivation materials,and the geometry that they cover or include, are selected for functionalcompatibility with the diode, including relatively high transparency soas not to interfere with the extraction of light from the diode orotherwise reduce its external efficiency.

In the drawings and specification there has been set forth a preferredembodiment of the invention, and although specific terms have beenemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being defined inthe claims.

1. A light emitting diode comprising: a submount structure; a bondingmetal on said submount structure; a barrier metal layer on said bondingmetal; an ohmic mirror layer encapsulated by said barrier metal layer;an epitaxial light emitting region on said encapsulated mirror; and alenticular surface at least partially on said epitaxial light emittingregion wherein said lenticular surface comprises silicon carbide.
 2. Alight emitting diode according to claim 1, said epitaxial light emittingregion comprising: a p-type gallium nitride epitaxial layer; an indiumgallium nitride active layer on said p-type layer; an n-type galliumnitride layer on said indium gallium nitride layer; and a bond pad tosaid n-type gallium nitride layer.
 3. A light emitting diode accordingto claim 1 wherein said ohmic mirror layer comprises a reflective metallayer and an ohmic metal layer.
 4. A light emitting diode according toclaim 3 wherein said reflective metal layer includes silver and saidohmic metal layer includes platinum.
 5. A light emitting diode accordingto claim 1 comprising a plurality of barrier metal layers on saidbonding metal.
 6. A light emitting diode according to claim 2 whereinsaid barrier metal layer is selected from the group consisting oftitanium, tungsten, tin and combinations and alloys of these metals. 7.A light emitting diode according to claim 1 wherein said active layer isin a mesa that includes a passivation layer that covers otherwiseexposed portions of said mirror, said barrier metal and said bondingmetal layer.
 8. A light emitting diode according to claim 7 wherein saidpassivation layer is selected from the group consisting ofstoichiometric silicon nitride, non-stoichiometric silicon nitride, andsilicon dioxide.
 9. A light emitting diode according to claim 1 whereinsaid bonding metal layer is selected from the group consisting of gold,tin and eutectic mixtures of gold and tin.
 10. A light emitting diodeaccording to claim 1 wherein said submount is selected from the groupconsisting of silicon, silicon carbide, gallium arsenide and coppertungsten.
 11. A light emitting diode according to claim 2 wherein saidbond pad includes an ohmic contact to said n-type gallium nitride layer.12. A light emitting diode according to claim 1 further comprising anohmic contact to said submount.
 13. A light emitting diode according toclaim 12 having a total dimension between and including said ohmiccontacts of no more than 150 microns.
 14. A light emitting diodeaccording to claim 1 further comprising a phosphor-containing structureadjacent said n-type gallium nitride layer for converting emissions fromsaid light emitting region into frequencies that will produce whitelight.
 15. A light emitting diode according to claim 10 wherein saidsubmount comprises single crystal silicon carbide and said siliconcarbide has a polytype selected from the 3C, 4H, 6H and 15R polytypes ofsilicon carbide.